Internal combustion engines have been used for over 100 years and researchers are always pursuing to increase its thermal efficiency and decrease its emissions. One of the measures to achieve high efficiency and low emissions for ordinary spark ignition engines at partial load operating conditions is downsizing the engine displacement, so that the throttle has to be opened wider and the pumping loss will be minimized. However, the engine displacement is required to be large at high load conditions in order to provide enough power. These different requirements of an engine displacement at different engine operating conditions lead to various challenges in engine design.
One of the solutions to realize this variable displacement engine is selectively shutting off several cylinders of an engine at partial load conditions. This means that, instead of reducing the air-fuel mixture charge by partially closing the throttle at partial load conditions, the stroke volume of the engine is reduced by disabling some of the working cylinders. At high load conditions, more working cylinders are activated to achieve higher power. This cylinder deactivation at low load conditions is conducted by cutting-off the fuel supply to the specifically selected cylinders. This will result in non-stoichiometric air-fuel ratio of exhaust gases in the exhaust system. Therefore, a conventional three-way catalyst converter is not enough to meet after-treatment requirements, and some other equipment is needed such as an expensive lean NOx trapper and/or a selective catalyst reduction (SCR) device. Another approach of using smaller displacement engines at low load conditions is turbo-charging the gasoline engine. This kind of engine configuration can make the engine more efficient at low load conditions because the engine displacement is small and the throttle has to be opened wider than that required for larger displacement engines. Therefore, its pumping loss is lower. At high load conditions, a turbo-charging system is used to increase the engine intake pressure so that the engine can trap more air-fuel mixture, resulting in more power. The disadvantages of downsizing the engine in addition to using a turbo-charger are increased complexities of the engine structure and control system, and a higher cost.
Variable stroke is another approach to achieve the variable engine displacement requirement. One type of variable stroke concept is longer expansion and exhaust strokes and shorter intake and compression strokes referred to as the Atkinson cycle. This cycle can be achieved by a Miller cycle, which delays the time at which the intake valve closes to reduce the effective compression stroke so that the compression stroke is shorter than the expansion stroke. Chadboume U.S. Pat. No. 1,326,129 and Clarke U.S. Pat. No. 4,044,629 described an extended expansion stroke engine. Mazda made this kind of Miller cycle engines. Honda developed a multiple leakage system to accomplish their variable stroke engines and was granted a series of patents for their inventions. Nakamura, et al. U.S. Pat. No. 6,575,128, Hiyoshi et al. U.S. Pat. No. 6,595,186, Aoyama, et al. U.S. Pat. No. 6,647,935, Nohara et al. U.S. Pat. No. 7,059,280, Tanaka et al. U.S. Pat. No. 7,234,424, Nohara et al. U.S. Pat. No. 6,550,436 and Yoshikawa et al. U.S. Pat. No. 8,261,703 described those inventions. Luis Marino Gonzalez invented an internal combustion engine design wherein a variable stroke was accomplished by using a gear set arrangement to connect the crankshaft and the piston connecting rods of the engine via offset bearing surfaces, given by U.S. Pat. No. 5,927,236 and U.S. Pat. No. 2012/0291755. The goal of all of these variable stroke inventions is to achieve a variation in the length of a piston stroke over a complete engine power cycle. In particular, these inventions seek to increase the expansion stroke during the expansion portion of the power cycle to increase the torque output, and to reduce the stroke and piston velocity during the intake portions of the cycle to decrease the pumping loss.
In addition to the variable stroke mechanisms mentioned above that change the strokes within one engine cycle, other inventions, for example Carl D. Heflev U.S. Pat. Nos. 7,270,092 and 5,335,632 included a mechanism that can change the stroke at different operating conditions by using an offset crankshaft mechanism. Particularly, it can realize a small displacement at low load conditions and a large displacement at high load conditions.
The compression ratio of an internal combustion engine should be as high as possible provided that no knocks occur for gasoline engines and that the peak cylinder pressure is within limit for diesel engines. Therefore, it is necessary that the compression ratio be adjustable based on load and speed conditions in order to improve efficiency within the entire engine operating condition range. In addition, if an engine uses different types of fuels, the compression ratio should also be adjusted based on the fuel type. In general, high-octane fuel allows engines to work at higher compression ratios in order to achieve better thermal efficiency.
In summary, the optimal engine displacement and compression ratio of an internal combustion engine both vary depending on the engine operating condition and the type of fuel used.